Microbial fuel cells (MFCs) are fuel cells which operate by using microorganisms that possess the ability to donate electrons to the anode of the fuel cell in order to produce electricity. Such microorganisms are known as exoelectrogenic organisms. Exoelectrogenic organisms can donate electrons to the anode in either of two ways: via mediators (e.g., the numerous dyes used in the art for this purpose) or in the absence of mediators (i.e. a mediator-less MFC).
An MFC contains an anode, a cathode, and a cation-selective permeable material (typically, a membrane) which separates the anodic solution from the cathodic solution. The anode and cathode are electrically connected (e.g., by a wire) outside of the electrode solutions. The microorganisms in contact with the anode oxidatively catabolize organic nutritive compounds, such as glucose, acetate, butyrate, methanol, ethanol, or the like, to produce electrons, protons (H+ ions), and oxidized organic material or carbon dioxide. The electrons are attracted to the anode and travel to the cathode. At the same time, the produced protons travel through the anodic solution and through the cation-selective permeable material to the cathode. At the cathode, oxygen gas (typically from air) reacts with the electrons and protons to produce water according to the reaction:O2+4H++4e−2H2O
A significant benefit of MFCs is their ability to produce electricity by environmentally-friendly and renewable means. Furthermore, MFCs can be fueled by waste products (e.g., waste water from sewage treatment or industrial waste), which are typically valueless and in need of degradation. MFCs can also be configured to produce hydrogen gas by, for example, providing an assistive anodic potential and eliminating oxygen from the cathode such that hydrogen can be produced at the cathode. In turn, there is great interest in hydrogen as a particularly environmentally clean fuel, such as used in ordinary hydrogen fuel cells. There is particular interest in producing hydrogen by environmentally-friendly means. Additionally, MFCs can also be configured to provide electrons to any reductive process requiring electrons, for example, by using a suitable electrode material for the cathode and passing the substrate requiring reduction through the cathode chamber. Some examples of reductive processes include nitrate reduction, uranium reduction and perchlorate reduction (Rabaey, K. et al. The ISME Journal 1, 9-18 (2007)). For at least the reasons given, MFCs continue to be the subject of intense research.
There are currently several problems which need to be overcome in order to make MFCs more commercially viable. One problem is that the exoelectrogenic microorganisms being used at the anode represent a small portion of the total amount of microorganisms operating at the anode. In other words, a significant portion of the microorganisms surrounding the anode are non-exoelectrogenic and do not contribute to production of electrical current. Another problem is that, typically, a significant portion of those microorganisms that operate by an exoelectrogenic mechanism do so by the indirect donation of electrons to one or more mediators. Both the low concentration of exoelectrogenic microorganisms and the low proportion of exoelectrogenic microorganisms which can operate by a direct electron transfer mechanism are factors that contribute to a low degree of efficiency in electrical power output. In addition, mediators are often expensive, toxic, and require regular replenishment. Elimination of their use would, therefore, provide many benefits.
Another problem with current MFCs is the occurrence of electrical power fluctuations. The fluctuations are typically caused by corresponding fluctuations in the amount of feed. Such fluctuations are detrimental to the commercial production of electricity. Yet, since such feed fluctuations are typical occurrences for most waste feed sources, a solution is needed to prevent MFC power fluctuations when operating under fluctuating feed conditions.
There is a need in the art for a microbial fuel cell which can operate more efficiently and thereby provide higher electrical power outputs. There is also a need in the art for a microbial fuel cell which can operate more reliably with minimal power fluctuations even during a period of time when a feed level is lowered well below a critical threshold.